A comment further up said they used Optically Stimulated Luminescence to date the sediment that tuned in to rock. You can read about the dating technique [here](https://www.baylor.edu/geosciences/index.php?id=955929#:~:text=Optically%20stimulated%20luminescence%20(OSL). The rock and the footprint must be the same age as the footprint was made while the sediment was still soft. It was then rapidly covered by more sediment, preserving the impression. Over hundreds of thousands of years, the sediment solidified under the pressure of the overlaying material.

(Edit: fixed link)